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Fix a couple of issues...

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1. The execute function should not be incrementing the PC register, as
this happens in other places.
2. The push_ and pop_stack functions were not considering that the data
should be in little-endian order.
This commit is contained in:
Peter Evans 2018-01-08 21:22:29 -06:00
parent e4049a6a11
commit b21b48cac0
1 changed files with 5 additions and 14 deletions
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19
src/mos6502.c
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@ -207,11 +207,11 @@ mos6502_push_stack(mos6502 *cpu, vm_16bit addr)
{
// First we need to set the hi byte, by shifting the address right 8
// positions and using the base offset of the S register.
vm_segment_set(cpu->memory, 0x0100 + cpu->S, addr >> 8);
vm_segment_set(cpu->memory, 0x0100 + cpu->S, addr & 0xff);
// Next we must record the lo byte, this time by using a bitmask to
// capture just the low end of addr, but recording it in S + 1.
vm_segment_set(cpu->memory, 0x0100 + cpu->S + 1, addr & 0xFF);
vm_segment_set(cpu->memory, 0x0100 + cpu->S + 1, addr >> 8);
// And finally we need to increment S by 2 (since we've used two
// bytes in the stack).
@ -231,9 +231,7 @@ mos6502_pop_stack(mos6502 *cpu)
// We need to use a bitwise-or operation to combine the hi and lo
// bytes we retrieve from the stack into the actual position we
// would use for the PC register.
return
(vm_segment_get(cpu->memory, 0x0100 + cpu->S) << 8) |
vm_segment_get(cpu->memory, 0x0100 + cpu->S + 1);
return vm_segment_get16(cpu->memory, 0x0100 + cpu->S);
}
/*
@ -354,18 +352,14 @@ void
mos6502_execute(mos6502 *cpu, vm_8bit opcode)
{
vm_8bit operand = 0;
int cycles, bytes;
int cycles;
mos6502_address_resolver resolver;
mos6502_instruction_handler handler;
// We want to know how many bytes this opcode and its operand
// consume. We know the opcode is one byte.
bytes = 1;
// The disassembler knows how many bytes each operand requires
// (maybe this code doesn't belong in the disassembler); let's use
// that to figure out the total number of bytes to skip.
bytes += mos6502_dis_expected_bytes(mos6502_addr_mode(opcode));
mos6502_dis_expected_bytes(mos6502_addr_mode(opcode));
// First, we need to know how to resolve our effective address and
// how to execute anything.
@ -403,9 +397,6 @@ mos6502_execute(mos6502 *cpu, vm_8bit opcode)
// something.
usleep(cycles * 100000);
// Update the counter to move beyond the opcode and its operand.
cpu->PC += bytes;
// Ok -- we're done! This wasn't so hard, was it?
return;
}